1. Field of the Invention
This invention relates to analog signal processing, and more particularly to programmable integrated circuits that perform analog processing, and include both user and manufacturer programmable capacitor arrays.
2. Description of the Related Art
Typically, analog integrated circuits are designed to receive one or more analog input signals, and process those signals by performing specific functions such as amplification, attenuation, filtering, integration, addition and subtraction. These functions usually dictate the topology of the analog integrated circuit. For example, the topologic arrangement of operational amplifiers and resistors are adjusted to provide either inverting or non-inverting gain. Every topology has specific noise, distortion and offset voltage sensitivities. Changing an analog circuit""s function often requires a change in the topology of the analog circuit, which in turn changes the noise, distortion and offset voltage characteristics of the circuit.
An analog integrated circuit with a programmable analog circuit block architecture permits change in a function of the analog circuit without changing the topology of the circuit elements, thereby reducing changes in voltage offset and distortion created by changes in topologies. Examples of such analog integrated circuit architectures can be found in U.S. Pat. No. 5,574,678, entitled xe2x80x9cContinuous Time Programmable Analog Block Architecture,xe2x80x9d by James L. Gorecki, which is incorporated herein by reference in its entirety.
In addition to such programmable analog circuits, another way to change an analog circuit""s function or performance characteristics is to vary the values of components in the circuit. In the case of an analog filter circuit, it is well known that the performance characteristics of the filter (e.g., the cutoff or corner frequency fc, the stop frequency fs, the passband ripple, and the stopband attenuation) depend generally on the RC product of the filter, and more particularly on the values of various resistors and capacitors in the filter. Thus, tunable or programmable filter circuits depend on the ability to adjust resistor and/or capacitor values.
When implementing integrated or monolithic programmable analog circuits, and particularly filters, processing and temperature variations can cause the values of various components in the circuit to vary by as much as 50%, which in turn can cause important circuit parameters, such as RC product, to vary by even greater amounts from their nominal design value. One conventional solution is to use integrated amplifiers in conjunction with high-precision, external resistors and capacitors. However, this approach is counter-productive to the goal of creating an integrated filter circuit, and increases both the size and the cost of the resulting filter.
Another solution is to provide for a fully integrated filter with an RC product that can be tuned to account for various process and temperature variations. One method for doing this, is to replace passive resistors with active resistors made from metal oxide field effect transistors (MOSFETs). For example, a feedback circuit can be implemented which continuously measures the RC product of the filter with reference to, for example, a clock or an external high-precision resistor. The outputs of the tuning circuit are then continuously applied to the MOSFET devices to set their resistances. A disadvantage to this technique is that the tuning circuits are very complex, sometimes more complex than the filter itself. Additionally, such tuning circuits usually require substantial amounts of power, and the variable range of the MOS resistors may not be sufficient to provide desired filter tunability. As an alternative to a tuning circuit and MOSFET resistors, arrays of passive resistors can be used with analog switches to afford programmability. However such an approach suffers from signal distortion due to the analog switches.
Still another alternative is to construct tunable filters by using tunable capacitor arrays and linear, passive resistors. Unfortunately, the value of capacitance per unit area in ordinary integrated circuit manufacturing processes is usually not well controlled. Therefore, if an accurate value of capacitance is needed for an integrated circuit capacitor, the manufacturer must xe2x80x9ctrimxe2x80x9d the capacitor as needed to compensate for the large variations inherent in ordinary integrated circuit manufacturing processes. One well known way of trimming the value of an integrated circuit capacitor is to provide an array in which a main capacitor of somewhat smaller than nominal capacitance is initially in parallel with a number of smaller xe2x80x9ctrimxe2x80x9d capacitors, which then can be removed from parallel connection with the main capacitor by trimming, for example by using a laser to cut electrical connections which directly or indirectly remove one or more of the trim capacitors from parallel connection to the main capacitor. This technique has the significant drawback that it requires that there be a capacitor trimming operation, which can add significantly to the cost of the product. Another problem with this technique is that it is not well suited to use in a user-programmable capacitor array because each user-programmable capacitance value has to be separately trimmed. Compounding this problem is the desire to have a wide range of programmable capacitances with high resolution (i.e., small steps between available capacitances).
Accordingly, it is desirable to have a user-programmable capacitor array that has high resolution and a wide range of available capacitances, while at the same time having an efficient and effective mechanism for a manufacturer to carry out trim operations. Additionally, it is desirable to have such a user-programmable capacitor array for use in programmable analog integrated circuits, particularly programmable filter circuits.
It has been discovered that a programmable capacitor array including a plurality of user-selectable, numerically weighted capacitors, each of which includes at least one fixed capacitor and one manufacturer-controlled trim capacitor, advantageously provides a variety of selectable capacitance values for a programmable analog integrated circuit. When coupled to a memory, for example a static memory, switches can be controlled that determine whether a particular fixed capacitor (user-selectable) or trim capacitor (manufacturer-selectable) is electrically coupled into the circuit. User access to those portions of memory controlling switches associated with the trim capacitors can be restricted via an I/O interface and security command. Such programmable capacitor arrays allow efficient implementation of user-programmable filter circuits where the user can conveniently program or reprogram a variety of filter parameters.
Accordingly, one aspect of the present invention provides a programmable capacitor array including a plurality of user-selectable capacitors. Each of the plurality of user-selectable capacitors is coupled in series with an associated user-controlled switch. Each of the plurality of user-selectable capacitors and associated user-controlled switches is coupled in parallel with each other of the plurality of user-selectable capacitors and associated user-controlled switches. At least one of the user-selectable capacitors includes at least one trim capacitor coupled in series with a manufacturer-controlled switch, and at least one fixed capacitor coupled in parallel with the at least one trim capacitor and the manufacturer-controlled switch.
In another aspect of the invention, a programmable analog circuit includes an amplifier having at least one input terminal and at least one output terminal, and a programmable capacitor array coupled between the at least one input terminal and the at least one output terminal. The programmable capacitor array includes a plurality of user-selectable capacitors. Each of the plurality of user-selectable capacitors is coupled in series with an associated user-controlled switch. Each of the plurality of user-selectable capacitors and associated user-controlled switches is coupled in parallel with each other of the plurality of user-selectable capacitors and associated user-controlled switches. At least one of the user-selectable capacitors includes at least one trim capacitor coupled in series with a manufacturer-controlled switch, and at least one fixed capacitor coupled in parallel with the at least one trim capacitor and the manufacturer-controlled switch.